I forget if it’s was 10.0, 10.1, or 10.2, but at least one previous version of OS X allowed you to choose the format of your screenshots. These days Apple has removed that feature. Why, I do not know.
That’s where SCIT comes in handy. It allows you to quickly, and easily change the default format to BMP, GIF, JPG, JP2, PCT, PDF, PNG, PSD, or TIF. Best of all… it’s free!